I was looking for a situation where it'd (b ^^*) a better move... Any ideas for a small board setting?
Namii: b is not possible before the liberty at c is filled. Check doubleko.
KonradScheffler: Well that more or less answers the question. Add a stone at c to the problem position. Now if white starts at a it will be seki, while if she starts at b it will be ko. This sort of ko (black standing to lose a lot, white only standing to lose a little) will be better (for white) than seki in the majority of cases.